A lot of wireless communication apparatuses use a frequency modulation method or a phase modulation method as a data modulation method. The wireless communication apparatus employing such modulation methods transmits and receives a carrier wave signal generated by increasing or decreasing the frequency of a carrier signal by a fine frequency according to data (baseband signal). Here, the frequency of the carrier signal is referred to as carrier frequency fc and the fine frequency is referred to as modulation frequency Δfc.
A conventional frequency modulation device having a structure as shown in FIG. 1 has been widely used. The frequency modulation device of FIG. 1 includes carrier signal generator 1 generating a carrier signal, digital-analog converter 2 converting a baseband signal which is a digital signal into an analog signal, bandwidth limiting filter 3 reducing harmonic components contained in the baseband signal output from digital-analog converter 2, and analog mixer 4 mixing the output signal of bandwidth limiting filter 3 with the carrier signal output from carrier signal generator 1, and increasing or decreasing the frequency of the carrier signal by a fine frequency according to the baseband signal to output a carrier wave signal.
The conventional frequency modulation device of FIG. 1 has disadvantages in that the circuit area increases due to bandwidth limiting filter 3, etc. and power consumption increases due to a large stationary current flowing through analog mixer 4.
A method for converting frequency of an input signal without using analog mixer 4 has been disclosed in e.g., International Publication Pamphlet No. 06/030905 (hereinafter, referred to as Patent reference 1). Patent reference 1 suggests a structure for shifting the phase of an input signal in every period using a phase interpolator to convert a frequency of the input signal. FIG. 2 shows a configuration example of the phase interpolator.
As illustrated in FIG. 2, CLK(i) and CLK(i+1) having the same frequency and different phases and a phase modulation control signal (digital signal) output from a control circuit (not shown) are input to phase interpolator 5.
Phase interpolator 5 divides time T equivalent to a phase difference between CLK(i) and CLK(i+1) by b:a(a+b=N) according to set value b input as the phase modulation control signal and outputs a signal delayed from CLK(i) by time (b/N)×T equivalent to set value b.
In detail, when a=4, b=3, the phase of CLK(i) is −135° with respect to a reference clock and the phase of CLK(i+1) is −180° with respect to the reference clock, since phase difference T between CLK(i) and CLK(i+1) is 45°, phase interpolator 5 outputs a signal delayed from CLK(i) by (3/7)×45°, i.e., a signal having a phase of −154°.
FIG. 3 illustrates a configuration example of a conventional frequency conversion device using phase interpolator 5 of FIG. 2. FIG. 3 illustrates the configuration described in above Patent reference 1.
As illustrated in FIG. 3, the conventional frequency conversion device includes m phase generation circuit 10, n phase generation circuit 20, and single phase clock generation circuit 30.
m phase generation circuit 10 outputs m phase clock signals having frequency fc/m and phase differences at equal intervals using a clock signal having frequency fc.
n phase generation circuit 20 includes n phase interpolators 5 shown in FIG. 2 and generates n phase clock signals having the same phase difference from the m phase clock signals generated in m phase generation circuit 10. That is, n phase generation circuit 20 outputs n phase clock signals having a period of (m/fc)×(1/n).
Single phase clock generation circuit 30 synthesizes the n phase clock signals output from n phase generation circuit 20 and outputs a single phase clock signal having a frequency of fc×n/m.
The frequency conversion device of FIG. 3 does not need analog mixer 4 and bandwidth limiting filter 3 shown in FIG. 1. In addition, when n and m are set to satisfy n/m=(fc+Δfc)/fc, a carrier signal having a frequency of fc can be converted into a carrier wave signal having a frequency of fc+Δfc.
In general, the carrier frequency used in the wireless communication apparatus ranges from a few hundred MHz to a few GHz and the modulation frequency used in the wireless communication apparatus ranges from a few ten KHz to a few MHz. Thus, when the frequency conversion device of FIG. 3 is employed in the wireless communication apparatus, if n and in are set to satisfy n/m=(fc+Δfc)/fc, n and m have very large values.
Accordingly, the size of the m phase generation circuit or the n phase generation circuit increases. In particular, the n phase generation circuit needs a lot of phase interpolators. Therefore, the frequency conversion device of FIG. 3 has disadvantages such as large circuit area or high power consumption.
As a result, the wireless communication apparatus, which is sorely in need of miniaturization or low power consumption, may not use the frequency conversion device of FIG. 3 as the frequency modulation device or as the phase modulation device.